Pressurized municipal water systems having a network of piping and faucets used to draw water from the system are in wide use and have been for well over a century. For almost as long, it has been recognized that in certain circumstances, pressure in the system (or in a localized part thereof) may diminish to a low level, e.g., below atmospheric pressure.
For example, if a user is drawing water from a faucet on, say, the second floor of a multi-story building and a fire stand pipe is opened at ground level, water pressure at the faucet may temporarily diminish to below atmospheric pressure. In a more common example, flushing a toilet may cause the pressure in a nearby faucet connected to the same part of the water main to fall dramatically. And users of shower baths often experience a sudden change in water temperature if a toilet is flushed or another sudden demand made on the water supply system.
It has also been long recognized that in the event of a drop in water pressure at a faucet or the like, there is a possibility that liquid in a container external to the system may be inadvertently drawn into the system. If the system distributes potable water (as is nearly always the case) and if the liquid in the container is impure in any way and is drawn into the system, the system may be contaminated.
Household kitchen sinks incorporate an early and very simple solution to this problem. To avoid drawing dirty water from the sink into a faucet in event of undue pressure drop, the faucet discharge opening is spaced above the elevation of the sink "flood level," i.e., the sink rim, by some distance, e.g., one inch (2.54 cm) or more. The resulting air gap between such opening and any liquid in the sink makes it impossible for sink water to be siphoned "backwards" into the faucet discharge opening, even if the liquid in the sink is at or overflowing the sink rim. This arrangement is described in American National Standards Institute (ANSI) Standard A112.1.2-1973 (R-1942) as promulgated by the American Society of Mechanical Engineers.
And for the same reason, it is common to space the end of the discharge pipe of a household dishwasher an inch (2.54 cm) or so above the line leading to the sewer. An example of a fitting suitable for that purpose is disclosed in U.S. Pat. No. 3,158,169 (Smith). Such fitting introduces an air gap between the upper waste pipe (from which dirty water flows) and the soil pipe below it which receives such dirty water. Fittings of this type are sometimes referred to as "vacuum breakers."
But that is not the end of the matter of preventing contamination of pressurized potable water systems. Other uses for water faucets are attended by the need to prevent backflow in more complex "operating environments." For example, one may wish to mix water from a faucet with a detergent concentrate in a container to form a more dilute washing solution. It is apparent that if one end of a hose is connected to the faucet and the other end immersed in the concentrate, a sudden drop in faucet water pressure might cause such concentrate to be drawn backwards through the hose and into the faucet and the water system.
And faucet water pressure need not drop to below atmospheric pressure to risk contaminating the water supply. If faucet water pressure falls to a level below that of the vessel containing, e.g., a dilute washing solution, the pressure in such vessel may cause the solution to back flow into the water system. Much the same result could occur if the vessel is elevated well above the faucet.
Back flow prevention techniques are often combined with venturi-type mixing devices, sometimes known as eductors. Such devices use a principle discovered by Daniel Bernoulli (1700-1782) and are used for applications involving mixing of two liquids, e.g., water and liquid chemical products. Examples of such devices are disclosed in U.S. Pat. No. 3,072,137 (McDougall) and U.S. Pat. No. 3,166,086 (Holmes), among several others.
The McDougall device has an upper water inlet coupling attached to a faucet and a side fitting for attaching a hose leading to a detergent container. When the faucet is turned on and when a valve button is depressed, detergent is drawn into the device and mixes with water. The dilute mixture is discharged out of the lower nozzle portion.
For back flow prevention, the McDougall device includes a chamber which is in air flow communication with passageways. Such passageways, which extend between the chamber and ambient air, maintain the pressure in the chamber at atmospheric pressure. Thus, if faucet pressure falls to below ambient pressure, ambient air rather than detergent or water/detergent mixture is drawn into such faucet. The McDougall device was found acceptable for so-called "low hazard" products with which faucet water was to be mixed.
Similar devices are disclosed in U.S. Pat. No. 4,697,610 (Bricker et al.); U.S. Pat. No. 5,159,958 (Sand) and U.S. Pat. No. 5,253,677 (Sand). The proportioner of the Bricker et al. patent was found generally satisfactory but in the commercial embodiment included an air gap of only about one-eighth inch (0.32 cm). It is understood that such proportioner does not meet ANSI standards.
A problem with the eductor disclosed in the Sand '958 patent is that when the water stream flows from the faucet across the air gap to the central opening and what is described as its conical sloped portion, not all of the water entered such central opening. To put it in other terms, there is a significant amount of lateral "splash" toward the slots leading to ambient air. To prevent water from splashing out of the air gaps, the eductor includes opaque, upwardly-extending tabs that substantially prevent such "splash-out." However, it is understood that persons charged with enforcing plumbing codes found this objectionable because the tabs prevented one from visually observing the presence of the air gap.
To address this objection, the eductor of the Sand '677 patent uses windows (which are understood to be transparent) radially inwardly offset from the eductor tubular body. But apparently this, too, was found objectionable because a plumbing inspector was prevented by the windows from physically passing an object or a finger through the air gap.
A characteristic of the mixing eductors disclosed in the Sand '958 and '677 patents relates to the collection chamber (the '958 patent) or the collection section (the '677 patent) of such eductors. Liquid which enters such chamber or section (presumably because the venturi section cannot accept the flow rate imposed upon it) is permitted to drain out to the solution tank rather than being bypassed to a separate drain or the like. And there is seemingly no way that a user of such an eductor can use it to fill a pail or other container with fresh water.
Another characteristic of the mixing eductors disclosed in the Sand '958 and '677 patents manifests itself if the vessel containing the chemical or the vessel containing the mixed solution are elevated above the eductor or if such vessels are pressurized for some reason. In either event, it appears that if water flow from the faucet is stopped, the chemical and/or solution "backs up" into the eductor air gap chamber and flow out of the slots to the floor.
Yet another characteristic of certain back flow prevention devices is that they must be oriented vertically. But sometimes vertical orientation is not practical or even possible. And yet another characteristic of such devices when combined with a mixing eductor is that reasonably-high water pressure is required to provide adequate downstream pressure drop to "drive" the eductor for satisfactory mixing.
Still another characteristic of certain back flow prevention devices is that they introduce undesirably high pressure drop in the water flow path. For example, the conical opening and converging nozzle mentioned in the Sand '958 and '677 patents, respectively, present relatively-long flow passages to a stream of water passing through such passages. And long flow passages impose higher pressure drops, leaving less pressure available for the mixing function.
Yet another characteristic of certain back flow prevention devices (the eductor of the Sand '677 patent for example) is that they are less-than-ideally suited for use with systems involving an apparatus containing concentrated liquids such as cleaning liquids. Such apparatus may be equipped with electromagnetic valves, the function of which is impaired by permitting air to enter the valves or the line leading to the valves. That can occur when the apparatus is mounted above the device and "loses prime." That is, by force of gravity, water flows backward in the line away from the apparatus.
And there is yet another matter of concern when conventional devices of the type not having an air gap are used to "feed" dispensing equipment such as equipment mixing water and concentrated cleaning liquids to obtain a cleaning solution. Commonly, faucets used by custodial staff have hot and cold water delivery pipes, each fitted with its own shutoff valve. The pipes are joined downstream of the valves and feed a common faucet. If both water delivery pipes are allowed to remain open, then water "cross-flow" between the hot and cold water pipes may occur.
A back flow prevention device which addresses and resolves some of the problems and shortcomings of earlier devices would be an important advance in the art.